Having searched t'internet for some square sockets, the best I could come up with was some 3/8" drive in 5/16" and 9/32" sizes. These were not a bad fit on the plumbing fittings I had, but really wanted some for a 1/4" drive ratchet.
My plumbing system was fitted by professionals, so the drain-down valves were in some pretty awkward places, being inaccessible with the standard key. With me not being a proper plumber, I was unhappy at having to use mole grips, pliers or even a 12" adjustable. ;)
The sockets were made from bits of brass. The drain-down valves seem to be machined in a couple of 'standard' sizes that would be covered by 7mm and 7.50mm square.
As I do not have a cnc mill, only a router, a fixture was required. That was a piece of 1/2" x 1" ali bar 100mm long. Holes drilled on the mill, transferred to lathe for the 15mm centre hole. A piece of lab worktop was zeroed on the router and the 2 x m8 mounting holes drilled. Needless to say the programmed square hole was not central. As I had drawn the worktop as part of the fixture it was a simple matter to centralise the square on the blank. The size of 8mm deep squares was changed by just altering the tool offset. The sockets are 1/4" one side and 7.0 or 7.50mm on the other.